1. Field of the Invention
The present invention relates to an improved column reactor device and processes for the purification of boron trichloride (BCl3). More particularly, the present invention relates to a device that minimizes silicon tetrachloride (SiCl4) formation during BCl3 production, and discloses processes for the removal of SiCl4 from the BCl3 product stream which may have been formed during the synthesis of BCl3.
2. Description of the State of the Art
Boron trichloride (BCl3) is a highly reactive compound packaged as a liquid under its own vapor pressure that has numerous diverse applications. It is used predominantly as a source of boron in a variety of manufacturing processes. For example, in the manufacturing of structural materials, BCl3 is the precursor for chemical vapor deposition (“CVD”) of boron filaments used to reinforce high performance composite materials. BCl3 is also used as a CVD precursor in the boron doping of optical fibers, scratch resistant coatings, and semiconductors. Some of the non-CVD applications of BCl3 are reactive ion etching of semiconductor integrated circuits and refining of metal alloys. In metallurgical applications, it is used to remove oxides, carbides, and nitrides from molten metals. In particular, BCl3 is used to refine aluminum and its alloys to improve tensile strength.
There are known a number of processes for the production of BCl3 for example, by chlorination of a borate ester, e.g., trimethyl borate, in a sealed tube. See, for example, U.S. Pat. No. 2,943,916. However, the most common technical process for the preparation of BCl3 is the reaction of a boron compound, such as boron carbide (B4C) with chlorine. In this process BCl3 can be prepared by passing chlorine over mixtures of boron carbide and optionally carbon, packed within a quartz column, which is heated to elevated temperatures of at least 800° C. to 1,200° C. Once the reaction is established, the reaction zone propagates slowly down the column generating BCl3 at the reaction zone. The chlorination reaction results in the formation of BCl3 having impurities such as unreacted chlorine (Cl2), hydrogen chloride (HCl), and phosgene (COCl2) which are generally removed from the raw BCl3 stream through distillation and/or other purification methods. However, trace amounts of silicon tetrachloride (SiCl4) are also produced and are much more difficult to remove from the product stream by the above described means due to its low volatility. The crude product, i.e., BCl3 containing the SiCl4 byproduct, is useful for some purposes; but, for many uses, SiCl4 is an undesirable impurity, e.g., when BCl3 is used as a precursor for high purity boron nitride. Therefore, its minimization during synthesis in the packed column reactor and its removal from the resulting BCl3 product stream is highly desirable.
Boron trioxide (B2O3) typically exists in boron carbide as an impurity with content varying from 1% (wt) to 5% (wt). Boron trioxide has a melting point temperature of about 450° C. or about 510° C. depending on its crystal structure. Hence, under the reaction condition as mentioned above, the impurity B2O3 in B4C melts and forms a liquid in the B4C chlorination process. The liquidized B2O3 in the process stream eventually forms deposits as the process temperature is below its melting point. The deposits may block the process stream flow as they are continuously accumulated after multiple reaction cycles. Typically, an activated carbon (such as charcoal) bed is loaded at the bottom of the reactor to adsorb liquidized B2O3. In the major section of the reactor, once the reaction is triggered, through induction heating, a porous carbon frame (graphite) is formed after boron is chlorinated and depleted from B4C. The presence of carbon (the carbon in the activated carbon bed and the carbon formed during the chlorination process) has a detrimental impact on BCl3 purity, i.e., carbon can enhance the chlorination of quartz (SiO2) at the B4C chlorination temperature of least 800° C. to 1,200° C. resulting in the formation of SiCl4, a highly undesirable impurity in BCl3, according to the reaction below:SiO2+C+2Cl2=SiCl4+CO2;ΔH° (1223 K)=−141.7 kJ/mol
Glow Discharge Mass Spectrometry (GDMS) analysis indicates that silicon also exists in boron carbide (0.38% (wt) in one batch of boron carbide sampled). Hence, the SiCl4 in the BCl3 stream may also be attributed to the silicon impurity in B4C (the source material of BCl3).
Therefore, there is a need to have a reactor for synthesizing BCl3 in the absence of a silicon source and/or a process for the removal of any SiCl4 impurities that may form during the synthesis processes.